Generalized Excess Noise Factor for Avalanche Photodiodes of Arbitrary Structure

The output current of a n avalanche photodiode (APD) fluctuates in the absence of light a s well a s in its presence. The noise in this current arises from three sources: randomness in the number and in the positions at which dark carrier pairs a r e generated, randomness in the photon arrival number, and randomness in the carrier multiplication process. The volume dark current in many multilayer and conventional APD’s, unlike the current arising from injected carriers, results from thermal or tunneling processes that generate electron-hole pairs randomly throughout the depletion region of the device. This results in a smaller mean multiplication and a larger excess noise factor than the usual values associated with carriers injected a t one edge of the depletion region. Photogenerated carriers produced by light incident on the depletion region a re also subject to this modified multiplication. We consider a generic model for a multilayer avalanche photodiode that admits arbitrary variation (with position) of the bandgap, dark generation rate, and ionization coefficients within each stage of the device. Expressions for the mean multiplication and excess noise factors for dark carriers alone, injected carriers alone, and for a n a r bitrary superposition of dark and injected carriers a r e derived for this general model. Special cases of our results reduce to well known expressions for the conventional APD, the separate absorptiodgrading/multiplication APD, the multiquantum-well APD, and the staircase APD.